[Design and power analysis: n and confidence intervals of means].
In this study, we analyzed the validity of the conventional 80% power. The minimal sample size and power needed to guarantee non-overlapping (1-alpha)% confidence intervals for population means were calculated. Several simulations indicate that the minimal power for two means (m = 2) to have non-overlapping CIs is .80, for (1-alpha) set to 95%. The minimal power becomes .86 for 99% CIs and .75 for 90% CIs. When multiple means are considered, the required minimal power increases considerably. This increase is even higher when the population means do not increase monotonically. Therefore, the often adopted criterion of a minimal power equal to .80 is not always adequate. Hence, to guarantee that the limits of the CIs do not overlap, most situations require a direct calculation of the minimum number of observations that should enter in a study.
García
José Fernando
JF
Facultad de Psicología, Universidad de Valencia, Valencia, Spain. fernando.garcia@uv.es
Pascual
Juan
J
Frías
María Dolores
MD
Van Krunckelsven
Dirk
D
Murgui
Sergio
S
D003198:Computer Simulation; D016001:Confidence Intervals; D006801:Humans; D008960:Models, Psychological; D011584:Psychology / Q000379:methods* / Q000706:statistics & numerical data*